The present invention relates to an image forming apparatus of a copying machine, printer or the like, and particularly to a fixing temperature control method of the fixing device of the image forming apparatus.
In an image forming apparatus based on an electrophotographic process, a fixing device comprising a heat roller and a pressure roller for bringing toner image-carrying recording paper in mechanical contact with this heating roller is generally provided with temperature detecting means for detecting temperature of heating roller surface. The controller of the image forming apparatus (hereinafter referred to as xe2x80x9ccontrollerxe2x80x9d) having received the detected temperature information exercises on/off control of such a heating source as a halogen lamp built in the heating roller in such a way that temperature of the heating roller surface is kept constant, thereby maintaining stable fixing performance.
The time required for heating operation (hereinafter referred to as xe2x80x9cwarm-upxe2x80x9d) where the heating source of a fixing device heats the heating roller up to the temperature that allows copying is set in such a way that the heating roller temperature reaches the level that allows copying immediately after the lapse of warm-up time, even under the severe conditions in terms of fixing performance if the environmental conditions are within the range guaranteed in the specification. This temperature that allows copying remains constant despite subsequent change in the conditions of using the image forming apparatus.
The temperature allowing copying equipped with this constant value is sufficient to ensure stable fixing performance, but involves problems in terms of saving energy.
Generally, in an image forming apparatus, the temperatures at various portions of the apparatus will be reduced if power is turned off for a long time. Once the copiable temperature has been reached by turning on power supply, the temperature inside the equipment including the pressure roller is gradually increased with the lapse of time by the heat source of the fixing device as long as power is kept turned on.
During the copying operation, the heat source is kept turned on to heat the heat roll. Almost all the heat is transferred to the recording paper carrying a toner image, and therefore the temperature of the pressure roller does not rise abruptly.
In the idling mode, the heat source keeps the heat roller at the copiable temperature using the power weaker than that in the copy mode, but the heat during this time does not cause an abrupt rise of pressure roller temperature.
The temperature of the heat roller surface is detected by a heat roller temperature sensor, and this surface temperature is kept at an appropriate temperature. However, it is difficult for the aforementioned temperature sensor to read changes in pressure roller temperature.
Rise of pressure roller temperature reduces the heat transferred to the pressure roller from the heat roll in the fixing mode. It can be said that, in order to ensure a certain level of fixing performance, the temperature on the heat roll surface immediately before fixing operation is saved more when a certain period of has passed after power was turned on, than immediately after power has been kept turned on for a long time.
However, keeping the control temperature on the heat roller constant despite the time when power is supplied to the image forming apparatus signifies that heating is excessive. This means that energy is wasted.
The first object of the present invention is to provide an energy-saving fixing temperature control method that detects or predicts the temperature of a pressure roller and minimizes the power consumption.
The Japanese Application Patent Laid-Open Publication No. Hei 02-154284 discloses a film fixing device comprising a fixing film to be transported, a heating body equipped with a heat generating resistor for melting toner on the sheet through a fixing film, and a pressure roller for gripping the sheet in mechanical contact with the heating body through the fixing film.
In order to ensure that toner is softened, melted and fixed with sheet firmly, and so-called high-temperature offset does not occur, the fixing device is preferred to exercise control so as to keep the temperature of the fixing roller and heating body to a level appropriate to fixing operation.
To achieve this purpose, the heat roller fixing device uses temperature detecting means comprising a thermister or the like to detect the temperature on the fixing roller surface. Based on the detected temperature, it exercises on/off control of the power supplied to the heat source.
The film fixing device disclosed in the aforementioned Official Publication uses a heat generating resistor of low heat capacity to accelerate the temperature rise. This is accompanied by the problem of the temperature of the heat generating resistor itself tending to be changed much by a slight change of electric power.
To solve this problem, the heating body of a film fixing device is designed in the following configuration: A heat generating resistor is installed on one side of the highly heat conductive substrate, and temperature detecting means is arranged on the other side. This temperature detecting means is brought in contact with the backside of the fixing film via the protective coating layer such as a glass or a ceramic layer. Big changes in temperature of the heat generating resistor are made gentle by installation of the temperature detecting means through such a good heat conductive substrate, and the temperature at the position in sliding contact with the fixing film is detected. Based on the detected temperature, the power to the heat generating resistor is placed under ON/OFF control.
Japanese Application Patent Laid-Open Publication No. Hei 10-69187 discloses a fixing device characterized by comprising magnetic flux generating means for inducing electricity and heating housed in the heat roll, and temperature detected means equipped with a temperature measuring element engaged with the portion where the induced current generated on the heat roller is the densest.
Using the aforementioned techniques, however, saving of power consumption has been insufficient especially from the viewpoint of energy saving.
From the viewpoint of energy saving in recent years, there has been a growing demand for a fixing device characterized by a very short warm-up time for allowing electric power to be kept turned off when not used. Not only that, from the viewpoint of saving natural resources and energy, it has become necessary to minimize the power consumption during the wait state (the state when the device is not used after an image forming apparatus power switch is turned on (called xe2x80x9cwarm-up)), and to reduce the time of heating the fixing device in the wait state in such a way that electric power is supplied to the fixing device only when the device is used, thereby enabling the device ready for operation in the shortest possible time.
The second object of the present invention is to provide an image forming apparatus characterized by excellent energy saving capability in the warm-up state and wait state (when the image forming apparatus is not used) and by reduced operation costs.
The first object of the present invention can be achieved by the following structures:
(1) A method of controlling a fixing temperature of a fixing device having a heat roller provided with a heat source and a first temperature detector provided at the vicinity of the heat roller, and having a pressure roller, which is in pressure contact with the heat roller, provided with a second temperature sensor at the vicinity of the pressure roller, comprising the steps of; detecting a temperature of the pressure roller by the second temperature detector; and varying a control temperature of the heat roller according to a detected temperature of the pressure roller.
(2) A method of controlling a fixing temperature of a fixing device having a heat roller provided with a heat source, and having a pressure roller, which is in pressure contact with the heat roller, comprising the steps of; measuring a elapsed time from a time of heating operation completion when the temperature of the heat roller reached at a temperature where fixing is capable; varying a control temperature of the heat roller according to the elapsed time.
(3) A method of controlling a fixing temperature of a fixing device having a heat roller provided with a heat source and a first temperature detector provided at the vicinity of the heat roller, and having a pressure roller, which is in pressure contact with the heat roller, provided with a second temperature sensor at the vicinity of the pressure roller, comprising the steps of; detecting a temperature of the pressure roller by the second temperature detector; measuring a elapsed time from a time of heating operation completion when the temperature of the heat roller reached at a temperature where fixing is capable; and varying a control temperature of the heat roller according to the detected temperature of the pressure roller the elapsed time from a time of heating operation completion.
(4) The method of controlling the fixing temperature according to the structures (1) through (3), further comprising the steps of; detecting a temperature and/or a humidity in an image forming apparatus including the fixing apparatus; and varying the control temperature of the heat roller according to the detected temperature and/or humidity in the image forming apparatus.
The second object of the present invention can be achieved by the following structures (5) through (9):
(5) An image forming apparatus comprising; a toner image forming section for forming a toner image on a photoreceptor; a transfer section for transferring the toner image from the photoreceptor on to a transfer sheet; and a fixing device for fixing the toner image on the transfer sheet, the fixing device having: a heat roller provided with a heat source and a first temperature detector at the vicinity of the heat roller; and a pressure roller, which is in pressure contact with the heat roller, provided a second temperature detector at the vicinity of the pressure roller; a controller for controlling the fixing temperature of the fixing device; wherein, a low power mode is established in a wait state of the image forming apparatus, and the controller varies a temperature setting for the heat roller in the low power mode according to the temperature of the pressure roller.
(6) An image forming apparatus comprising: a toner image forming section for forming a toner image on a photoreceptor; a transfer section for transferring the toner image from the photoreceptor on to a transfer sheet; and a fixing device for fixing the toner image on the transfer sheet, the fixing device having: a heat roller provided with a heat source and a first temperature detector at the vicinity of the heat roller; a pressure roller, which is in pressure contact with the heat roller; a controller for controlling the fixing temperature of the fixing device; and a temperature estimation device for estimating a temperature of the pressure roller based on an operation history of the fixing device; wherein, the controller controls the fixing temperature according to the estimated temperature of the pressure roller.
(7) An image forming apparatus comprising: a toner image forming section for forming a toner image on a photoreceptor; a transfer section for transferring the toner image from the photoreceptor on to a transfer sheet; and a fixing device for fixing the toner image on the transfer sheet, the fixing device having: a heat roller provided with a heat source and a first temperature detector at the vicinity of the heat roller; and a pressure roller, which is in pressure contact with the heat roller; a power supply device for supplying power to the fixing device; a time measuring device for measuring a time period of supplying power to the fixing device; a temperature estimation device for estimating a temperature rise rate of the heat roller, when the power supply device have supplied a power to the fixing device during a warm-up mode, based on the heat roller temperature and the time period of supplying power; and a controller for controlling the fixing temperature of the fixing device; wherein, the controller controls the fixing temperature according to the estimated temperature rise rate of the heat roller.
(8) The image forming apparatus according to (7), wherein a low power mode is established in a wait state of the image forming apparatus, and according to a temperature rise rate calculated by the temperature estimation device, the controller adjusts a setting temperature of the heat roller in the low power mode.
(9) An image forming apparatus comprising: a toner image forming section for forming a toner image on a photoreceptor; a transfer section for transferring the toner image from the photoreceptor on to a transfer sheet; and a fixing device for fixing the toner image on the transfer sheet, the fixing device having: a heat roller provided with a heat source and a first temperature detector at the vicinity of the heat roller; and a pressure roller, which is in pressure contact with the heat roller; a controller for controlling the fixing temperature of the fixing device; wherein, when a setting temperature of the heat roller in a fixing mode is changed from A to B, a setting temperature of the heat roller in a low power mode is adjusted to C, which is previously set corresponding to the setting temperature B in the fixing mode.